Basically, if you mix two different frequencies (and this is the outcome of mixing audio with different pitch), you will get a multitude of frequencies.
For example, if the original audio contains a frequency of 1000 hz at some point, this will be represented as 800 Hz and 700 Hz. If you now mix these frequencies, you will get the sum of these frequencies (1500 Hz) as well as the difference (100 Hz) in the resulting output. Now with this multitude of frequencies (100, 700, 800 and 1500 Hz) which are all on different levels, the signal is really distorted and the source signal is hard to reconstruct (probably what all those evildoers will want).
As long as you have a single frequency you want to reconstruct, you might use up- and downmixing and then filtering out the leftover parts of the signal to regain the original frequency. This is what a radio usually does to get the audio signal from the radio frequency.
If you have a mixture of frequencies that are changing all the time, it is nearly impossible to reconstruct the original audio, as you can only guess and try and filter, as you already did.